ERIC K. PRITCHARD The Tube Sound and Tube Emulators $0 RE TUBES MAGIC? IS THERE really a difference be- tween tubes and tran- I ea sistors? Some hear the warmthA and appreciate the full body of the tube sound, and others deride the thought. Is the magic of I. I. - the tube sound more than mere / ea nostalgia? A recording engineer, ....„1 .4,Aa. e1 00 14 •111 Russell 0. Hamm, could hear the Relative lame Level itoletive ler.a Level difference. Determined to find and Figure 1.Distortion components explain the difference, he began Figure 3.Distortion components for two-stage triode amplifier. testing microphone preamplifiers for multi-stage capacitor-coupled of various technologies. His fa- transistor amplifier. mous paper, "Tubes Versus Tran- sistors—Is There an Audible Dif- ference?" [1], shows that the 10 harmonic structures in overdrive 3 re N I 3 r• conditions for different technolo- gies are quite different, almost like 20 10 fingerprints. I. -___.......i..4_7" More recently, an electronics en- 14 ea gineer, Eric Pritchard, started Relative brat Level - 14 down the circuitous path to bring Retellv• tepee Leve1 Figure 2..Distortion components the two worlds together and to give Figure 4.Distortion components for two-stage pentode amplifier.. solid state the character of tubes. for multistage transformer- The elusive tube sound has finally coupled transistor amplifier. succumbed to an intensive re- search and development program TUBES AND THE TUBE that has produced solid-state tube SOUND emulators and tube emulator cir- In retrospect, the tube has many cuits [2]. The effort began nearly 3 re f technically superior aspects, it is seven years ago with the search for fairly linear, its operational pa- / a solid state guitar amplifier that rameters do not vary badly, at least / sounded like tubes. After snaking / for tubes of yesteryear. / through myths and theories, the vs.......... 14. 7/ -- It operates reasonably without research turned to emulating the feedback. Its gain-bandwidth // . tubes, both triodes and pentodes. product is not low, about 8 MHz for 1 111 ell a 12AX7 in a generic audio stage, Relalive beer Level compared with 3 MHz for typical Russell Hamm's work provided a Figure 5.Distortion components audio operational amplifiers. Tube test and an independent source of for monolithic operational parameters do not vary as badly as data to correlate the operation of amplifier with hybrid output semiconductor parameters. the triode tube emulator. stage. They do not have the widely varying input voltage threshold of nearly obsolete, and they are rap- FETs. Tubes, in fact, may be the idly becoming unavailable. Eric K. Pritchard is president of most natural amplifier. Unfortu- Deja Vu Audio, Berkeley Springs, nately, they are large, they are The tube sound has been so elu- WV 25411 fragile, they are microphonic, they sive that many consider it to be 26 db January/February 1994 drift, they burn out, they are mysterious and beyond the capa- bility of modern instrumentation. age. Other devices, pentodes and This is partly true. The tube sound all semiconductors, have a sharply is often more subtle than the oscil- rising current in the saturation re- loscope display. Low level harmon- gion that then bends over into a ics are not visible, but are audible. constant current region. The triode Distortion meters do not consider plate characteristic insures that individual harmonics, but our ears for moderate-to-high impedance do. Standard audio tests do not tell circuits the grid conduction always the whole story. limits the negative excursion of the Russell Hamm moved beyond plate for any reasonable load line. the Total Harmonic Distortion This contrasts with pentodes and measurement and developed a test semi-conductors which saturate that separated different types of for small loads. This is evident in the pentode preamplifier which microphone preamplifiers: triode, Figure 6.Complete Tube Emula- has the second harmonic rising pentode, transistor, and opera- tor for a 12AX7. tional amplifier. This test meas- substantially later than it rises in ured and plotted the percentage of the triode preamplifier. each harmonic as a function of This is quite evident in operational overdrive. amplifier based preamplifier, Fig- THE TRIODE TUBE The harmonic character of these ure 5. A transistor amplifier pat- EMULATOR preamplifiers, Figures 1 through 5 ent, in an attempt to sound like The full triode emulation circuit, [1], are quite different, virtual fin- tubes, has reduced the extent of shown in Figure 6, has all of the gerprints of the various technolo- feedback to avoid these problems needed features: grid, plate, and gies and their respective circuits. [6]. Tubes, being more natural am- cathode terminals, grid-to-cathode The triode curves, Figure 1, show plifiers, need less feedback and significant second harmonic gener- consequently do not generate as conduction, grid-to-plate capaci- tance, gain, and the non-linear net- ated by the bias shifting in the cou- much of these high, harsh, discor- works. The gain is created by two pling capacitor created by grid con- dant harmonics. operational amplifiers such as the duction. The pentode curves, Tubes, particularly triodes pro- dual op amp, 0PA2604. This op- Figure 2, show the grid conduction duce significant second harmonics, erational amplifier has a sufficient delayed by the plate load curve go- Figure 1. The second harmonic has gain-bandwidth product, about 10 ing through the saturation region two sources: the non-linearity of MHz, to simulate audio tube cir- well below the knee into the high tube characteristics and the inter- cuits. The grid terminal drives the plate resistance region. The semi- action of the coupling capacitor and negative input of the first op amp conductor preamp curves, Figures the grid-to-cathode diode [7]. The to produce the needed inversion, 3 through 5, show no equivalence non-linear characteristics are the while the second op amp creates to grid conduction. The operational plate resistance and the gain. The the feedback for the first. R1 deter- amplifier, Figure 5, shows the plate resistance and the gain pro- mines the proportion of distortion rapid rise in distortion created by duce harmonics at all signal levels. in the voltage gain. R2 then deter- high gains and substantial feed- These harmonics are superseded mines the voltage gain. back. by those created by the interaction The second harmonic provides of the coupling capacitor, grid-to- The choice of voltage gains is lim- punch in contrast to the blanket of cathode diode, and clipping. The ited by the selection of the grid con- the third harmonic [1]. Conse- grid-to-cathode diode charges the duction components. The grid quently, these Figures show that coupling capacitor when the grid drive circuit cannot produce so the triode, Figure 1 initially pro- conducts. The resulting change in much grid-to-cathode voltage that vides a blanketed punch that fades charge creates an offset that shifts the first operational amplifier goes into a lot of punch. The pentode is the bias from quiescent conditions. into negative saturation, yet a primarily blanketed with a little This bias shift alters the duty cycle large output voltage swing is desir- punch. Semiconductors vary from of the resulting waveform. A duty able. Thus, there is an engineering quite blanketed to completely cycle which is not 50-50 produces choice between the diode voltage blanketed. even harmonics. The two harmonic drop, the power supply voltage, The next remarkable feature is sources plus the low feedback com- and the gain. the generation or rather the lack of bine to produce harmonics that oc- The reason why the tube emula- high order harmonics. The high or- cur over a wide range of input. This tor does not behave as its opera- der harmonics, especially odd ones, range is far wider than found in tional amplifier is that operational put a discordant edge on the signal. typical transistor or semiconductor amplifier is kept out of negative A prime source of high order har- designs. By comparison then, they saturation by the grid-to-cathode monics is feedback. Although feed- distort too fast. diode and the positive saturation back corrects for amplifier errors, The grid conduction plus the effects are not transmitted by an it also attempts to correct for power unique plate characteristics give output resistance that becomes ef- supply limits. The feedback slams triodes the soft clip characteristic. fectively infinite. Of course, this re- the output against the power sup- Triode plate resistance is unique quires that the tube emulator plate ply rails and creates sharp corners because the plate current sweeps supply be at or below the op komqed/A.Ionuorqp amp pool that translate into high harmonics. upward with increasing plate volt- saturation level. L of input levels and frequencies show a good match at various over- drives, frequencies and loads, see Figures 11-20. A close examination of these figures show slight differ- ences in the curvature in some por- 1:1 ( tions of some of the waveforms. This is reflected in slight differ- ences in harmonic levels. Harmonic analysis showed that the tube emulator erred in the di- rection of a more ideal tube, Figure 9.0utput (plate) charac- teristics of a 12AX7 triode tube slightly more rounded waveforms Figure 7.Low-Noise Tube Emu- emulator. and consequently less intense high lator for a 12AX7. Horz: 5 volts I div. order harmonics. Vert: 0.5 milliamps I div, Step: .05 volts. The low-noise implementation, shown in Figure 7, is viable for flat response stages. A difference be- 4 LV 30K tween these circuits is the feed- back connection between the two operational amplifiers is attenu- ated by R6 and R7 so that the noise of the second op amp is also attenu- ated.